Optical semiconductor element and optical semiconductor module
By extending the electrical length to the second optical modulator and using a third pad with termination resistors, the optical modulation waveform is improved by synchronizing signal timings and equalizing electrical lengths, addressing timing mismatches and resonance issues in optical modulators.
Patent Information
- Authority / Receiving Office
- WO · WO
- Patent Type
- Applications
- Current Assignee / Owner
- MITSUBISHI ELECTRIC CORP
- Filing Date
- 2025-01-15
- Publication Date
- 2026-07-23
Smart Images
Figure JP2025000999_23072026_PF_FP_ABST
Abstract
Description
Optical Semiconductor Device and Optical Semiconductor Module
[0001] The present disclosure relates to an optical semiconductor device and an optical semiconductor module.
[0002] An optical semiconductor device in which a first optical modulator and a second optical modulator are arranged in series and the second optical modulator modulates the output light of the first optical modulator is used. However, when a differential electrical signal is simultaneously input to the first optical modulator and the second optical modulator, the timing at which the output light of the first optical modulator enters the second optical modulator and the timing at which the electrical signal enters the second optical modulator are shifted, resulting in a decrease in bandwidth and deterioration of the eye opening height of the optical modulation waveform. Therefore, it has been proposed to make the electrical length from the signal input pad to the second optical modulator longer than the electrical length from the signal input pad to the first optical modulator to delay the electrical signal entering the second optical modulator signal (see, for example, Patent Document 1).
[0003] Japanese Patent No. 4698888
[0004] However, when the electrical length from the pad to the second optical modulator is increased, the electrical length between the termination resistor wire-connected to the pad and the second optical modulator also increases. Therefore, since the electrical length to the termination resistor differs between the differentials, a peak occurs in the frequency characteristics of the small signal transmission due to resonance, and there is a problem that noise and jitter occur in the optical modulation waveform.
[0005] The present disclosure has been made to solve the above-described problems, and an object thereof is to obtain an optical semiconductor device and an optical semiconductor module capable of improving an optical modulation waveform.
[0006] The optical semiconductor device according to this disclosure comprises a substrate, first and second optical modulators formed on the substrate, and first, second and third pads formed on the substrate, wherein the second optical modulator modulates the output light of the first optical modulator, and each of the first and second optical modulators has a first conductivity type semiconductor layer, a second conductivity type semiconductor layer, and an absorption layer formed between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer, wherein the first conductivity type semiconductor layer of the first optical modulator and the second conductivity type semiconductor layer of the second optical modulator are connected, the first pad is connected to the second conductivity type semiconductor layer of the first optical modulator, the second pad is connected to the first conductivity type semiconductor layer of the second optical modulator, the electrical length from the second pad to the second optical modulator is longer than the electrical length from the first pad to the first optical modulator, and the third pad is connected between the second pad and the second optical modulator.
[0007] In this disclosure, the electrical length from the second pad to the second optical modulator is made longer than the electrical length from the first pad to the first optical modulator. This delays the electrical signal entering the second optical modulator, allowing the timing of the output light from the first optical modulator entering the second optical modulator to be synchronized with the timing of the electrical signal entering the second optical modulator. As a result, the bandwidth and the eye aperture height of the optical modulation waveform can be improved. Furthermore, a third pad is connected between the second pad and the second optical modulator. By connecting termination resistors to both the first and third pads, the electrical length to the termination resistors can be made the same between the differentials. This prevents the occurrence of peaks in the frequency characteristics of small-signal transmission due to resonance, thereby improving the optical modulation waveform.
[0008] This is a plan view showing an optical semiconductor element according to Embodiment 1. This is a cross-sectional view of the first optical modulator along line I-II in Figure 1. This is a cross-sectional view of the second optical modulator along line III-IV in Figure 1. This is a plan view showing an optical semiconductor module according to Embodiment 1. This is a cross-sectional view of the optical semiconductor element according to Embodiment 1 along the waveguide. This is a schematic diagram showing an optical semiconductor module according to Embodiment 1. This is a plan view showing an optical semiconductor module according to Comparative Example 1. This is a schematic diagram showing an optical semiconductor module according to Comparative Example 1. This is a diagram showing the electrical differential reflection of Comparative Example 1. This is a diagram showing the response (relative value) of the first light to the first electrical signal of Comparative Example 1. This is a diagram showing the response (relative value) of the second light to the second electrical signal of Comparative Example 1. This is a diagram showing the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals of Comparative Example 1. This is a diagram showing the optical modulation waveform of Comparative Example 1. This is a diagram showing the electrical differential reflection of Embodiment 1. This is a diagram showing the response (relative value) of the first light to the first electrical signal of Embodiment 1. This is a diagram showing the response (relative value) of the second light to the second electrical signal of Embodiment 1. This figure shows the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals of Embodiment 1. This figure shows the optical modulation waveform of Embodiment 1. This is a schematic diagram showing a modified example 1 of the optical semiconductor module according to Embodiment 1. This is a schematic diagram showing an optical semiconductor module according to Comparative Example 2. This figure shows the electrical differential reflection of Comparative Example 2. This figure shows the response (relative value) of the first light to the first electrical signal of Comparative Example 2. This figure shows the response (relative value) of the second light to the second electrical signal of Comparative Example 2. This figure shows the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals of Comparative Example 2. This figure shows the optical modulation waveform of Comparative Example 2. This figure shows the electrical differential reflection of Modified Example 1 of Embodiment 1. This figure shows the response (relative value) of the first light to the first electrical signal of Modified Example 1 of Embodiment 1. This figure shows the response (relative value) of the second light to the second electrical signal of Modified Example 1 of Embodiment 1. This figure shows the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals of Modified Example 1 of Embodiment 1. This figure shows the optical modulation waveform of Embodiment 1. This is a plan view showing a modified example 2 of the optical semiconductor module according to Embodiment 1.This is a schematic diagram showing a modified example 2 of the optical semiconductor module according to Embodiment 1. This is a diagram showing the extinction curve of the optical modulator in modified example 2 of Embodiment 1. This is a diagram showing the optical modulation waveform when the voltage applied to the optical modulator is changed in modified example 2 of Embodiment 1. This is a plan view showing a modified example 3 of the optical semiconductor element according to Embodiment 1. This is a plan view showing a modified example 3 of the mounting substrate according to Embodiment 1. This is a cross-sectional view showing a modified example 3 of the optical semiconductor module according to Embodiment 1. This is a plan view showing a modified example 4 of the optical semiconductor module according to Embodiment 1. This is a schematic diagram showing a modified example 4 of the optical semiconductor module according to Embodiment 1. This is a plan view showing an optical semiconductor module according to Embodiment 2. This is a plan view showing a modified example 1 of the optical semiconductor module according to Embodiment 2. This is a plan view showing a modified example 2 of the optical semiconductor module according to Embodiment 2. This is a plan view showing a modified example 3 of the optical semiconductor module according to Embodiment 2. This is a plan view showing a modified example 4 of the optical semiconductor element according to Embodiment 2. This is a plan view showing a modified example 4 of the mounting substrate according to Embodiment 2. This is a cross-sectional view showing a modified example 4 of the optical semiconductor module according to Embodiment 2. This is a plan view showing an optical semiconductor module according to Embodiment 3. This is a plan view showing a modified example of the optical semiconductor module according to Embodiment 3.
[0009] The optical semiconductor element and optical semiconductor module according to the embodiment will be described with reference to the drawings. The same or corresponding components are denoted by the same reference numerals, and repetition of the description may be omitted.
[0010] Embodiment 1 Figure 1 is a plan view showing an optical semiconductor device according to Embodiment 1. The optical semiconductor device 100 is an electro-absorption modulator integrated laser diode (EML) in which a distributed feedback type semiconductor laser 1 and an electro-absorption type first optical modulator 2a and a second optical modulator 2b are monolithically integrated on a semi-insulating InP substrate 3. The output terminal of the semiconductor laser 1 and the input terminal of the first optical modulator 2a are connected via a transparent waveguide 4a. The output terminal of the first optical modulator 2a and the input terminal of the second optical modulator 2b are connected via a transparent waveguide 4b.
[0011] The semiconductor laser 1 has a p electrode 5a and an n electrode 5b. The p electrode 5c is connected to the p side of the first optical modulator 2a, and a common electrode 5d is connected to the n side. The n electrode 5e is connected to the n side of the second optical modulator 2b, and a common electrode 5d is connected to the p side. The first pad 6a, the second pad 6b, and the third pad 6c are formed on a semi-insulating InP substrate 3. The first pad 6a is connected to the p electrode 5c. The second pad 6b is connected to the n electrode 5e. The third pad 6c is connected between the second pad 6b and the n electrode 5e. The first pad 6a, the second pad 6b, and the third pad 6c are arranged on the same side with respect to the waveguide.
[0012] The electrical length between the second pad 6b and the n electrode 5e is longer than the electrical length between the first pad 6a and the p electrode 5c. The electrical length between the third pad 6c and the n electrode 5e is the same as the electrical length between the first pad 6a and the p electrode 5c.
[0013] Figure 2 is a cross-sectional view of the first optical modulator along line I-II in Figure 1. On a semi-insulating InP substrate 3, the semiconductor stacked structure of the first optical modulator 2a consists of an n-InGaAs contact layer 8a, an n-InP cladding layer 8b, an absorption layer 8c, a p-InP cladding layer 8d, and a p-InGaAs contact layer 8e, stacked in that order. The absorption layer 8c is a multiple quantum well structure of InGaAsP. An insulating film 9 covers the top surface of the semi-insulating InP substrate 3 and the side surfaces of the waveguide of the first optical modulator 2a.
[0014] An opening is formed in the insulating film 9 on the waveguide of the first optical modulator 2a, and the p electrode 5c is connected to the p-InGaAs contact layer 8e through this opening. The common electrode 5d is connected to the n-InGaAs contact layer 8a. The first pad 6a is placed on the semi-insulating InP substrate 3 via the insulating film 9 and is connected to the p-InGaAs contact layer 8e via the p electrode 5c.
[0015] Figure 3 is a cross-sectional view of the second optical modulator along the line III-IV in Figure 1. The semiconductor stack structure of the second optical modulator 2b is the same as that of the first optical modulator 2a. An opening is formed in the insulating film 9 on the waveguide of the second optical modulator 2b, and a common electrode 5d is connected to the p-InGaAs contact layer 8e through this opening. That is, the n-InGaAs contact layer 8a of the first optical modulator 2a and the p-InGaAs contact layer 8e of the second optical modulator 2b are connected by the common electrode 5d. The n electrode 5e is connected to the n-InGaAs contact layer 8a. The second pad 6b is placed on the semi-insulating InP substrate 3 via the insulating film 9 and is connected to the n-InGaAs contact layer 8a via the n electrode 5e.
[0016] Figure 4 is a plan view showing an optical semiconductor module according to Embodiment 1. A laser current line 11, ground electrodes 12a and 12b, a first modulation signal line 13a, a second modulation signal line 13b, and termination resistor pads 14a and 14b are formed on the mounting substrate 10. The mounting substrate 10 may be a submount. The ground electrodes 12a and 12b are grounded. The first modulation signal line 13a and the second modulation signal line 13b receive differential signals from input terminals 13ain and 13bin. Termination resistors 15a and 15b are mounted on the mounting substrate 10. Termination resistor 15a is connected between the termination resistor pad 14a and the ground electrode 12b. Termination resistor 15b is connected between the termination resistor pad 14b and the ground electrode 12b.
[0017] The optical semiconductor element 100 is mounted on a mounting substrate 10. The p electrode 5a of the semiconductor laser 1 of the optical semiconductor element 100 is connected to the laser current line 11 by a wire 16a. The n electrode 5b of the semiconductor laser 1 is connected to the ground electrode 12a by a wire 16b. The output terminal 13aout of the first modulation signal line 13a is connected to the first pad 6a of the optical semiconductor element 100 by a wire 16c. The output terminal 13bout of the second modulation signal line 13b is connected to the second pad 6b by a wire 16d. The lengths of wires 16c and 16d are the same. The first pad 6a is connected to the termination resistor pad 14a by a wire 16e. Therefore, the first termination resistor 15a is connected to the first pad 6a. The third pad 6c is connected to the termination resistor pad 14b by a wire 16f. Therefore, the second termination resistor 15b is connected to the third pad 6c. Wire 16e and wire 16f are the same length.
[0018] The drive circuit 17 supplies differential signals to the input terminal 13ain of the first modulation signal line 13a and the input terminal 13bin of the second modulation signal line 13b. The electrical signals input to input terminal 13ain and the electrical signals input to input terminal 13bin are in opposite phases, but there is no delay difference. The drive circuit 17 is formed outside the mounting substrate 10, but it may also be formed on the mounting substrate 10.
[0019] Figure 5 is a cross-sectional view of the optical semiconductor device according to Embodiment 1 along the waveguide. On the semi-insulating InP substrate 3, a semiconductor laser 1, a transparent waveguide 4a, a first optical modulator 2a, a transparent waveguide 4b, and a second optical modulator 2b are arranged in order along the direction of light propagation.
[0020] [Correction based on Rule 91 24.01.2025] A positive-phase signal is input to the input terminal 13ain of the first modulation signal line 13a and supplied to the first optical modulator 2a via the first pad 6a. A negative-phase signal is input to the input terminal 13bin of the second modulation signal line 13b and supplied to the second optical modulator 2b via the second pad 6b. The positive-phase signal and the negative-phase signal are differential voltages with opposite phases. The differential voltage applied between the first pad 6a and the second pad 6b causes the first optical modulator 2a and the second optical modulator 2b to operate differentially. The first optical modulator 2a modulates the light emitted from the semiconductor laser 1. The second optical modulator 2b modulates the output light of the first optical modulator 2a. Therefore, if the size of each modulator is the same, a larger optical modulation amplitude (extinction ratio) can be obtained compared to a single optical modulator without a decrease in cutoff frequency.
[0021] The time from when the electrical signal is input to the second optical modulator 2b until the optical signal reaches the output terminal of the second optical modulator 2b is shorter than the time from when the electrical signal is input to the first optical modulator 2a until the optical signal reaches the output terminal of the second optical modulator 2b. This difference in optical delay is at least the delay due to the distance Lo1 from the output terminal of the first optical modulator 2a to the input terminal of the second optical modulator 2b, and at most the delay due to the distance Lo2 from the input terminal of the first optical modulator 2a to the output terminal of the second optical modulator 2b.
[0022] Due to this difference in optical delay, if electrical signals are input to both the first optical modulator 2a and the second optical modulator 2b simultaneously, the timing of the optical signal input to the second optical modulator 2b and the electrical signal will not match, causing the amplitude of the optical signal output from the output terminal of the second optical modulator 2b to decrease. To prevent this, an electrical delay difference is generated by the electrical length Le of the wiring between the second pad 6b and the third pad 6c, delaying the timing of the electrical signal input to the second optical modulator 2b compared to the timing of the electrical signal input to the first optical modulator 2a. This synchronizes the timing of the optical signal input to the second optical modulator 2b and the electrical signal.
[0023] The delay difference due to light is calculated as: path distance difference ÷ speed of light in a vacuum × refractive index of light. The delay difference due to electricity is calculated as: path distance difference ÷ speed of light in a vacuum × (relative permittivity)^0.5, since the relative permeability of InP is approximately 1. In the case of InP, the refractive index of light is approximately 3.2, the effective relative permittivity is approximately 8, and (relative permittivity)^0.5 is less than 3. Therefore, if the line length is the same, the electrical signal travels about 1 to 2 times faster. The minimum delay difference due to light is the delay difference due to distance Lo1, so if the speeds of the electrical signal and the optical signal are the same, Lo1 ≤ Le must be satisfied. On the other hand, the maximum delay difference due to light is the delay difference due to distance Lo2, so if the electrical signal travels twice as fast as light, 1 / 2 x Le ≤ Lo2 must be satisfied. Therefore, Lo1 ≤ Le ≤ 2 x Lo2 must be satisfied.
[0024] Figure 6 is a schematic diagram showing an optical semiconductor module according to Embodiment 1. A first electrical signal S1 supplied from a positive signal source to a first modulation signal line 13a and a second electrical signal S2 supplied from a negative signal source to a second modulation signal line 13b are in opposite phases to each other. The first optical modulator 2a receives laser light L emitted from the semiconductor laser 1, modulates it according to the first electrical signal S1, and outputs a first optical light L1. The second optical modulator 2b receives the first optical light L1, modulates it according to the second electrical signal S2, and outputs a second optical light L2. An electrical delay difference 18 is inserted between the second pad 6b and the third pad 6c.
[0025] The first modulation signal line 13a and the second modulation signal line 13b have independent lines that do not interfere with each other. In this case, the even-mode characteristic impedance Ze and the odd-mode characteristic impedance Zo are Zo = Ze. The larger K = (Ze - Zo) / (Ze + Zo), the higher the degree of coupling, and if K < 0.1, the degree of coupling is relatively small, so it is defined as an independent line.
[0026] Next, the effects of this embodiment will be explained in comparison with Comparative Example 1. Figure 7 is a plan view showing the optical semiconductor module according to Comparative Example 1. Figure 8 is a schematic diagram showing the optical semiconductor module according to Comparative Example 1. The comparative example lacks the third pad 6c, and the second pad 6b is connected to the termination resistor pad 14b by a wire 16f. As a result, an electrical delay difference 18 is inserted between the second optical modulator 2b and the termination resistor 15b.
[0027] Assuming an optical delay difference of 2 ps between the first optical fiber L1 and the second optical fiber L2, the simulation results for the cases where the electrical delay difference 18 is 0 ps and 2 ps will be explained. The simulation was performed using NRZ, but it may also be performed using PAM. For the independent transmission line, the circuit simulation was performed with Zo = Ze = 50Ω (K = 0).
[0028] [Correction based on Rule 91 24.01.2025] Figure 9 shows the electrical differential reflection of Comparative Example 1. When the ports of the input terminal 13ain of the first modulated signal line and the input terminal 13bin of the second modulated signal line are set to 1 and 2, the electrical differential reflection S11dd becomes (S(1,1) - S(1,2) - S(2,1) + S(2,2)) / 2. Figure 10 shows the response (relative value) of the first light to the first electrical signal of Comparative Example 1. The response of the first light L1 shows no peaks in the frequency response characteristics and no bandwidth reduction.
[0029] Figure 11 shows the response (relative value) of the second light to the second electrical signal in Comparative Example 1. In Comparative Example 1, the electrical length between the second optical modulator 2b and the termination resistor 15b is large, so a peak occurs when the electrical delay difference is 2 ps.
[0030] Figure 12 shows the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals in Comparative Example 1. When the electrical delay difference is 0 ps, the phase of the light input to the second optical modulator 2b does not match the phase of the electrical signal, so the bandwidth deteriorates. When the electrical delay difference is 2 ps, the phases of both are matched, but a peak occurs in the frequency characteristics due to resonance.
[0031] Figure 13 shows the optical modulation waveform of Comparative Example 1. The first optical fiber L1 has no noise, jitter, or deterioration of eye aperture height. When the electrical delay difference is 0 ps, the eye aperture height of the second optical fiber L2 deteriorates due to bandwidth degradation. When the electrical delay difference is 2 ps, noise and jitter occur in the second optical fiber L2.
[0032] Figure 14 shows the electrical differential reflection of Embodiment 1. Figure 15 shows the response (relative value) of the first light to the first electrical signal of Embodiment 1. Figure 16 shows the response (relative value) of the second light to the second electrical signal of Embodiment 1. Figure 17 shows the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals of Embodiment 1. No peak occurs in the frequency characteristics of Embodiment 1.
[0033] Figure 18 shows the optical modulation waveform of Embodiment 1. When the electrical delay difference is 0 ps, the eye aperture height of the second optical element L2 deteriorates, similar to Comparative Example 1. On the other hand, when the electrical delay difference is 2 ps, the phase of the light input to the second optical modulator 2b matches the phase of the electrical signal, thus improving the optical modulation waveform.
[0034] As described above, in this embodiment, the electrical length from the second pad 6b to the second optical modulator 2b is made longer than the electrical length from the first pad 6a to the first optical modulator 2a. This delays the electrical signal entering the second optical modulator 2b, allowing the timing of the output light from the first optical modulator 2a entering the second optical modulator 2b to be synchronized with the timing of the electrical signal entering the second optical modulator 2b. As a result, the bandwidth and the eye aperture height of the optical modulation waveform can be improved. Furthermore, the third pad 6c is connected between the second pad 6b and the second optical modulator 2b. By connecting termination resistors 15a and 15b to the first pad 6a and the third pad 6c, respectively, the electrical lengths to the termination resistors 15a and 15b can be made the same between the differentials. This prevents the occurrence of peaks in the frequency characteristics of small signal transmission due to resonance, thereby improving the optical modulation waveform.
[0035] Furthermore, in this embodiment, a delay difference is applied to the electrical signals input to the first optical modulator 2a and the second optical modulator 2b. However, by using independent transmission lines, the system is less susceptible to waveform distortion caused by reflected waves resulting from the delay difference.
[0036] Modification 1 of Embodiment 1 Figure 19 is a schematic diagram showing Modification 1 of the optical semiconductor module according to Embodiment 1. The first modulation signal line 13a and the second modulation signal line 13b have a coupling line that interferes with each other. In this case, Zo < Ze. If K ≥ 0.1, the degree of coupling is relatively large, so it is defined as a coupling line. The other configurations are the same as in Embodiment 1.
[0037] Next, the effects of Modification 1 of Embodiment 1 will be explained in comparison with Comparative Example 2. Figure 20 is a schematic diagram showing an optical semiconductor module according to Comparative Example 2. Comparative Example 2 does not have a third pad 6c, and the second pad 6b is connected to the termination resistor pad 14b by a wire 16f. As a result, an electrical delay difference 18 is inserted between the second optical modulator 2b and the termination resistor 15b.
[0038] [Correction based on Rule 91 24.01.2025] Assuming that the optical delay difference between the first light L1 and the second light L2 is 2 ps, the simulation results for the cases where the electrical delay difference 18 is 0 ps and 2 ps are explained. Circuit simulations were performed in the coupling line with Zo = 50 Ω and Ze = 79 Ω (K = approximately 0.225).
[0039] Figure 21 shows the electrical differential reflection of Comparative Example 2. Figure 22 shows the response (relative value) of the first light to the first electrical signal of Comparative Example 2. The response of the first light L1 shows no peak in the frequency response characteristics and no bandwidth reduction. Figure 23 shows the response (relative value) of the second light to the second electrical signal of Comparative Example 2. Figure 24 shows the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals of Comparative Example 2. Similar to Comparative Example 1, a peak occurs in the frequency characteristics due to resonance when the electrical delay difference is 2 ps. Figure 25 shows the optical modulation waveform of Comparative Example 2. When the electrical delay difference is 0 ps, the eye aperture height of the second light L2 deteriorates due to bandwidth degradation. When the electrical delay difference is 2 ps, noise and jitter occur in the second light L2.
[0040] Figure 26 shows the electrical differential reflection of Modification 1 of Embodiment 1. Figure 27 shows the response (relative value) of the first light to the first electrical signal in Modification 1 of Embodiment 1. Figure 28 shows the response (relative value) of the second light to the second electrical signal in Modification 1 of Embodiment 1. Figure 29 shows the response (relative value) of the second light to the simultaneous differential input of the first and second electrical signals in Modification 1 of Embodiment 1. No peak occurs in the frequency characteristics of Modification 1 of Embodiment 1. Figure 30 shows the optical modulation waveform of Embodiment 1. When the electrical delay difference is 0 ps, the eye aperture height of the second light L2 deteriorates, similar to Comparative Example 2. On the other hand, when the electrical delay difference is 2 ps, the optical modulation waveform can be improved.
[0041] Furthermore, since applying an electrical delay difference between differentials causes the phase of the reflected wave to differ between differentials, adding an electrical delay difference to a coupling line that interferes with each other is usually considered taboo. However, by matching the timing (phase) of the optical signal and electrical signal input to the second optical modulator 2b, the interference is canceled out, thus reducing noise and jitter even when a coupling line is used. Using a coupling line also allows for a reduction in the space between signal wiring, enabling space savings.
[0042] [Correction based on Rule 91 24.01.2025] In addition, for multi-channel applications, a single optical semiconductor module outputs multiple optical modulation signals. In this case, multiple optical semiconductor elements 100 are mounted on a single mounting substrate 10, or multiple sets of semiconductor lasers 1 and first optical modulators 2a and second optical modulators 2b are mounted on a single optical semiconductor element 100. Therefore, since the space for electrical signal wiring is limited, coupling lines that are advantageous for space saving are effective.
[0043] Modification Example 2 of Embodiment 1 FIG. 31 is a plan view showing a modification example 2 of the optical semiconductor module according to Embodiment 1. FIG. 32 is a schematic view showing a modification example 2 of the optical semiconductor module according to Embodiment 1. The optical semiconductor element 100 further has a common electrode pad 19 connected to the common electrode 5d. The common electrode pad 19 is connected to the ground electrode 12a of the mounting substrate 10 by a wire 16g. Thereby, the n-type semiconductor of the first optical modulator 2a and the p-type semiconductor of the second optical modulator 2b are grounded. Thereby, the ratio of the voltage Voff1 applied to the first optical modulator 2a and the voltage Voff2 applied to the second optical modulator 2b can be freely set, and the optical modulation waveforms such as the cross point, extinction ratio, Tr (Rise Time), and Tf (Fall Time) can be optimized.
[0044] FIG. 33 is a diagram showing the extinction curve of the optical modulator of Modification Example 2 of Embodiment 1. FIG. 34 is a diagram showing the optical modulation waveform when the voltage applied to the optical modulator is changed in Modification Example 2 of Embodiment 1. When |Voff1| and |Voff2| are increased, the cross point where the rising line and falling line of light cross decreases, and the extinction ratio increases. When |Voff1| is increased, the cross point of the first light L1 decreases. When |Voff2| is increased, the cross point of the second light L2 decreases. The position of the cross point near the middle (50% of the cross point) between the H (mark) level and the L (space) level is the most favorable for the eye to open. It is desirable that not only the second light L2 but also the first light L1 has a better cross point. In the above extinction curve, the cross point of the first light L1 becomes 50% around Voff1 = -2.0V. When Voff1 = -2.0V, the cross point of the second light L2 becomes 50% around Voff2 = +1.4V. That is, for optimal cross point operation, it is necessary to make |Voff1| > |Voff2|, which can be realized by grounding the n-type semiconductor of the first optical modulator 2a and the p-type semiconductor of the second optical modulator 2b.
[0045] Modification 3 of Embodiment 1 Figure 35 is a plan view showing modification 3 of the optical semiconductor element according to Embodiment 1. The parts enclosed by the dashed lines of the p electrode 5a and n electrode 5b of the semiconductor laser 1 are the connection pads 5ap and 5bp, respectively. To prevent the parts other than the first pad 6a, second pad 6b, third pad 6c and connection pads 5ap and 5bp from contacting the wiring of the mounting substrate 10, only these pads are made to protrude, or the surfaces other than the pads are covered with an insulator.
[0046] Figure 36 is a plan view showing a modified example 3 of the mounting substrate according to Embodiment 1. A laser current line 11, a ground electrode 12, a first modulation signal line 13a, a second modulation signal line 13b, and termination resistor wirings 20a and 20b are formed on the mounting substrate 10. The ground electrode 12 is grounded. The output terminal 13aout of the first modulation signal line 13a is connected to one end of the termination resistor wiring 20a. Termination resistors 15a and 15b are mounted on the mounting substrate 10. The termination resistor 15a is connected between the other end of the termination resistor wiring 20a and the ground electrode 12. The termination resistor 15b is connected between the other end of the termination resistor wiring 20b and the ground electrode 12. The lengths of the first modulation signal line 13a and the second modulation signal line 13b are the same. The lengths of the termination resistor wiring 20a and the termination resistor wiring 20b are the same.
[0047] Figure 37 is a cross-sectional view showing a modified example 3 of the optical semiconductor module according to Embodiment 1. Figure 37 corresponds to a cross-sectional view along line I-II in Figures 35 and 36. The optical semiconductor element 100 is mounted on the mounting substrate 10 by a conductive bonding material 21 such as solder using a junction-down method. Specifically, the connection pad 5ap of the p electrode 5a of the semiconductor laser 1 of the optical semiconductor element 100 is connected to the laser current line 11. The connection pad 5bp of the n electrode 5b of the semiconductor laser 1 of the optical semiconductor element 100 is connected to the ground electrode 12. The first pad 6a of the optical semiconductor element 100 is connected to the output terminal 13aout of the first modulation signal line 13a. The second pad 6b is connected to the output terminal 13bout of the second modulation signal line 13b. The third pad 6c is connected to one end of the termination resistor wiring 20b. The optical semiconductor element 100 can be easily mounted using the junction-down method. Other configurations and effects are the same as in Embodiment 1.
[0048] [Correction based on Rule 91, 24.01.2025] Modified Example 4 of Embodiment 1 FIG. 38 is a plan view showing a modified example 4 of the optical semiconductor module according to Embodiment 1. FIG. 39 is a schematic view showing a modified example 4 of the optical semiconductor module according to Embodiment 1. In modified example 4, the directions of the p-side and n-side of the optical modulator are opposite to those in Embodiment 1. Specifically, an n electrode 5e is connected to the n-side of the first optical modulator 2a, and a common electrode 5d is connected to the p-side. The first pad 6a is connected to the n electrode 5e. A p electrode 5c is connected to the p-side of the second optical modulator 2b, and a common electrode 5d is connected to the n-side. The second pad 6b is connected to the p electrode 5c. The third pad 6c is connected between the second pad 6b and the p electrode 5c. In this case, the positive and negative of the signal source and the positive and negative of the voltages Voff1 and Voff2 are opposite to those in Embodiment 1. Other configurations are the same as those in Embodiment 1, and the same effects as those in Embodiment 1 can be obtained.
[0049] Embodiment 2 FIG. 40 is a plan view showing an optical semiconductor module according to Embodiment 2. The second modulation signal line 13b has an extension 13bex that is bent horizontally on the output end 13bout side. The electrical length from the input end 13bin to the output end 13bout of the second modulation signal line 13b is longer than the electrical length from the input end 13ain to the output end 13aout of the first modulation signal line 13a by the amount of this extension 13bex. As a result, the electrical signal entering the second modulation signal line 13b is delayed, so that the timing when the output light of the first optical modulator 2a enters the second optical modulator 2b can be matched with the timing when the electrical signal enters the second optical modulator 2b. As a result, the bandwidth and the eye opening height of the optical modulation waveform can be improved. Also, by wire-connecting terminating resistors 15a and 15b to the first pad 6a and the second pad 6b, respectively, the electrical lengths up to the terminating resistors 15a and 15b can be made the same between the differentials. This prevents a peak from occurring in the frequency characteristics of small-signal transmission due to resonance and improves the optical modulation waveform. Other configurations and effects are the same as those in Embodiment 1.
[0050] Modification 1 of Embodiment 2 Figure 41 is a plan view showing Modification 1 of the optical semiconductor module according to Embodiment 2. The second modulation signal line 13b is bent outward at the output end, and moves further away from the first modulation signal line 13a as it approaches the output end. As a result, the electrical length of the second modulation signal line 13b becomes longer than the electrical length of the first modulation signal line 13a, and the same effect as in Embodiment 2 can be obtained.
[0051] Modification 2 of Embodiment 2 Figure 42 is a plan view showing Modification 2 of the optical semiconductor module according to Embodiment 2. In Modification 1, the distance between the second modulation signal line 13b and the first modulation signal line 13a widens at the bent portion, so the capacitance between the lines increases. The impedances Z0 and Zo of the lines are determined by √(L / C), where L is the inductance and C is the capacitance. Therefore, as the capacitance increases, the impedance decreases. The impedances of the first modulation signal line 13a and the second modulation signal line 13b need to be matched between the input terminal, the parallel line, the bent line, and the output terminal. However, if the impedance of the second modulation signal line 13b is high at the bent portion, the impedance will be misaligned with that of other parts. Therefore, in Modification 2, the widths of the first modulation signal line 13a and the second modulation signal line 13b are widened towards the output terminals 13aout and 13bout, respectively. By widening the line width in this way and increasing the capacitance, the impedance can be matched with that of other parts. The other configurations and effects are the same as in Modification 1.
[0052] Modification 3 of Embodiment 2 Figure 43 is a plan view showing modification 3 of the optical semiconductor module according to Embodiment 2. The first modulation signal line 13a and the second modulation signal line 13b are arranged at an angle to the waveguides of the first optical modulator 2a and the second optical modulator 2b, with their input ends tilted toward the semiconductor laser 1. As a result, the electrical length of the second modulation signal line 13b is longer than the electrical length of the first modulation signal line 13a, so that the same effect as in Embodiment 2 can be obtained.
[0053] Modification 4 of Embodiment 2 Figure 44 is a plan view showing Modification 4 of the optical semiconductor element according to Embodiment 2. In the semiconductor laser 1, the parts enclosed by the dashed lines of the n electrode 5b and p electrode 5a are the connection pads 5ap and 5bp, respectively. To prevent the parts other than the first pad 6a, second pad 6b, third pad 6c and connection pads 5ap and 5bp from contacting the wiring of the mounting substrate 10, only these pads are made to protrude, or the surfaces other than the pads are covered with an insulator.
[0054] Figure 45 is a plan view showing a modified example 4 of the mounting substrate according to Embodiment 2. A laser current line 11, a ground electrode 12, a first modulation signal line 13a, a second modulation signal line 13b, and termination resistor wirings 20a and 20b are formed on the mounting substrate 10. The ground electrode 12 is grounded. The output terminal 13aout of the first modulation signal line 13a is connected to one end of the termination resistor wiring 20a. The output terminal 13bout of the second modulation signal line 13b is connected to one end of the termination resistor wiring 20b. Termination resistors 15a and 15b are mounted on the mounting substrate 10. The termination resistor 15a is connected between the other end of the termination resistor wiring 20a and the ground electrode 12. The termination resistor 15b is connected between the other end of the termination resistor wiring 20b and the ground electrode 12. The length of the second modulation signal line 13b is longer than the length of the first modulation signal line 13a by the amount of the extension portion 13bex. The lengths of the termination resistor wiring 20a and the termination resistor wiring 20b are the same.
[0055] Figure 46 is a cross-sectional view showing a modified example 4 of the optical semiconductor module according to Embodiment 2. Figure 46 corresponds to a cross-sectional view along line I-II in Figures 44 and 45. The optical semiconductor element 100 is mounted on the mounting substrate 10 using a junction-down configuration. Specifically, the connection pad 5ap of the p electrode 5a of the semiconductor laser 1 of the optical semiconductor element 100 is connected to the laser current line 11. The connection pad 5bp of the n electrode 5b of the semiconductor laser 1 of the optical semiconductor element 100 is connected to the ground electrode 12. The first pad 6a of the optical semiconductor element 100 is connected to the output terminal 13aout of the first modulation signal line 13a. The second pad 6b is connected to the output terminal 13bout of the second modulation signal line 13b. Other configurations and effects are the same as in Embodiment 2. The optical semiconductor element 100 can be easily mounted using the junction-down configuration.
[0056] Embodiment 3 Figure 47 is a plan view showing an optical semiconductor module according to Embodiment 3. In this embodiment, the electrical length of the first modulation signal line 13a and the electrical length of the second modulation signal line 13b are the same, but the electrical length of the second wire 16 is longer than that of the first wire 16. As a result, the electrical signal entering the second modulation signal line 13b is delayed, so that the timing of the output light from the first optical modulator 2a entering the second optical modulator 2b and the timing of the electrical signal entering the second optical modulator 2b can be matched. As a result, the bandwidth and the eye aperture height of the optical modulation waveform can be improved. In addition, by wire-connecting termination resistors 15a and 15b to the first pad 6a and the second pad 6b, respectively, the electrical length to the termination resistors 15a and 15b can be made the same between the differentials. This prevents the occurrence of peaks in the frequency characteristics of small signal transmission due to resonance and improves the optical modulation waveform. Other configurations and effects are the same as in Embodiment 1.
[0057] Figure 48, a modified example of Embodiment 3, is a plan view showing a modified example of the optical semiconductor module according to Embodiment 3. In Embodiment 3, the centerlines of the first modulation signal line 13a and the second modulation signal line 13b coincide with the centerlines of the first pad 6a and the second pad 6b. On the other hand, in the modified example, the centerlines of the first modulation signal line 13a and the second modulation signal line 13b are offset outward from the centerlines of the first pad 6a and the second pad 6b. As a result, the distance between the second modulation signal line 13b and the second pad 6b is greater than the distance between the first modulation signal line 13a and the first pad 6a. Therefore, the electrical length of the second wire 16 is longer than the electrical length of the first wire 16, and the same effect as in Embodiment 3 can be obtained.
[0058] 2a First optical modulator, 2b Second optical modulator, 3 Semi-insulating InP substrate, 6a First pad, 6b Second pad, 6c Third pad, 8a n-InGaAs contact layer (first conductivity semiconductor layer), 8b n-InP cladding layer (first conductivity semiconductor layer), 8c Absorption layer, 8d p-InP cladding layer (second conductivity semiconductor layer), 8e p-InGaAs contact layer (second conductivity semiconductor layer), 10 Mounting substrate, 12, 12a, 12b Ground electrodes, 13a First modulation signal line, 13b Second modulation signal line, 13ain, 13bin Input terminals, 15a First termination resistor, 15b Second termination resistor, 16c Wire (first wire), 16d Wire (second wire), 17 Drive circuit, 100 Optoelectronic semiconductor element
Claims
1. An optical semiconductor element comprising: a substrate; first and second optical modulators formed on the substrate; and first, second and third pads formed on the substrate, wherein the second optical modulator modulates the output light of the first optical modulator; each of the first and second optical modulators has a first conductivity type semiconductor layer, a second conductivity type semiconductor layer, and an absorption layer formed between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer; the first conductivity type semiconductor layer of the first optical modulator and the second conductivity type semiconductor layer of the second optical modulator are connected; the first pad is connected to the second conductivity type semiconductor layer of the first optical modulator; the second pad is connected to the first conductivity type semiconductor layer of the second optical modulator; the electrical length from the second pad to the second optical modulator is longer than the electrical length from the first pad to the first optical modulator; and the third pad is connected between the second pad and the second optical modulator.
2. The optical semiconductor element according to claim 1, characterized in that the electrical length between the second pad and the third pad is greater than or equal to the distance from the output terminal of the first optical modulator to the input terminal of the second optical modulator, and less than or equal to the distance from the input terminal of the first optical modulator to the output terminal of the second optical modulator.
3. An optical semiconductor module comprising: a mounting substrate having first and second modulation signal lines for inputting differential signals from an input terminal; an optical semiconductor element according to claim 1 or 2 mounted on the mounting substrate; and first and second termination resistors mounted on the mounting substrate, wherein the output terminal of the first modulation signal line is connected to the first pad, the output terminal of the second modulation signal line is connected to the second pad, the first termination resistor is connected to the first pad, and the second termination resistor is connected to the third pad.
4. The optical semiconductor module according to claim 3, characterized in that the electrical length from the third pad to the second termination resistor is the same as the electrical length from the first pad to the first termination resistor.
5. A mounting substrate having first and second modulation signal lines that input differential signals from the input terminal, an optical semiconductor element mounted on the mounting substrate, and first and second termination resistors mounted on the mounting substrate, wherein the optical semiconductor element has a substrate, first and second optical modulators formed on the substrate, and first and second pads formed on the substrate, the second optical modulator modulates the output light of the first optical modulator, each of the first and second optical modulators has a first conductivity type semiconductor layer, a second conductivity type semiconductor layer, and an absorption layer formed between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer, the first conductivity type semiconductor layer of the first optical modulator and the second conductivity type semiconductor layer of the second optical modulator are connected, the first pad is connected to the second conductivity type semiconductor layer of the first optical modulator, the second pad is connected to the first conductivity type semiconductor layer of the second optical modulator, and the output terminal of the first modulation signal line is connected to the first pad. An optical semiconductor module characterized in that the output terminal of the second modulation signal line is connected to the second pad, the first termination resistor is connected to the first pad, the second termination resistor is connected to the second pad, and the electrical length of the second modulation signal line from the input terminal to the output terminal is longer than the electrical length of the first modulation signal line from the input terminal to the output terminal.
6. The optical semiconductor module according to claim 5, characterized in that the second modulation signal line moves further away from the first modulation signal line as it approaches the output terminal.
7. The optical semiconductor module according to claim 6, characterized in that the width of the first and second modulation signal lines increases towards the output terminal.
8. The optical semiconductor module according to claim 5, characterized in that the first and second modulation signal lines are arranged obliquely to the waveguides of the first and second optical modulators.
9. The optical semiconductor module according to any one of claims 3 to 8, characterized in that the optical semiconductor element is mounted on the mounting substrate in a junction-down configuration.
10. A mounting substrate having first and second modulation signal lines that input differential signals from the input terminal, an optical semiconductor element mounted on the mounting substrate, and first and second termination resistors mounted on the mounting substrate, wherein the optical semiconductor element has a substrate, first and second optical modulators formed on the substrate, and first and second pads formed on the substrate, the second optical modulator modulates the output light of the first optical modulator, each of the first and second optical modulators has a first conductivity type semiconductor layer, a second conductivity type semiconductor layer, and an absorption layer formed between the first conductivity type semiconductor layer and the second conductivity type semiconductor layer, the first conductivity type semiconductor layer of the first optical modulator and the second conductivity type semiconductor layer of the second optical modulator are connected, the first pad is connected to the second conductivity type semiconductor layer of the first optical modulator, the second pad is connected to the first conductivity type semiconductor layer of the second optical modulator, and the output terminal of the first modulation signal line is connected to the first pad by a first wire. An optical semiconductor module characterized in that the output terminal of the second modulation signal line is connected to the second pad by a second wire, the first termination resistor is connected to the first pad, the second termination resistor is connected to the second pad, and the electrical length of the second wire is longer than the electrical length of the first wire.
11. The optical semiconductor module according to claim 10, characterized in that the distance between the second modulation signal line and the second pad is greater than the distance between the first modulation signal line and the first pad.
12. The optical semiconductor module according to any one of claims 5 to 11, characterized in that the electrical length from the second pad to the second termination resistor is the same as the electrical length from the first pad to the first termination resistor.
13. The optical semiconductor module according to any one of claims 3 to 12, characterized in that the electrical length from the input terminal of the second modulation signal line to the second optical modulator is longer than the electrical length from the input terminal of the first modulation signal line to the first optical modulator.
14. The optical semiconductor module according to any one of claims 3 to 13, wherein the optical semiconductor element further has a common electrode connecting the first conductivity type semiconductor layer of the first optical modulator and the second conductivity type semiconductor layer of the second optical modulator, the mounting substrate further has a grounded ground electrode, and the common electrode is connected to the ground electrode.
15. The optical semiconductor module according to any one of claims 3 to 14, characterized in that the first and second modulation signal lines have independent lines that do not interfere with each other.
16. The optical semiconductor module according to any one of claims 3 to 14, characterized in that the first and second modulated signal lines have a coupling line that interferes with each other.
17. The optical semiconductor module according to any one of claims 3 to 16, further comprising a drive circuit that supplies the differential signal to the input terminals of the first and second modulation signal lines.